1. Field of the Invention
The present invention relates to a method for setting a temperature of a sensor element.
2. Description of the Related Art
A multitude of sensor elements for detecting at least one characteristic of a gas in a measuring gas chamber are known from the related art. In principle, this may be any arbitrary characteristic of the gas, for example, a physical and/or chemical characteristic of the gas. In particular, the present invention is described hereafter with reference to sensor elements for detecting a fraction, i.e., for example, a partial pressure and/or a percentage, of at least one gas component in the gas. The gas component may be oxygen in particular. Other gas components may also alternatively or additionally be detected, however, for example, nitrogen oxides, hydrocarbons, or other gas components. However, the present invention is not restricted to the detection of gas components, but rather in principle, alternatively or additionally, other characteristics of the gas may also be detected.
In particular, sensor elements which are based on the use of at least one solid electrolyte, i.e., an ion-conducting solid, for example, an oxygen ion-conducting solid, are used to detect gas components. Such solid electrolytes may be prepared, for example, based on zirconium dioxide, for example, yttrium-stabilized zirconium dioxide and/or scandium-doped zirconium dioxide. Such sensor elements are used, for example, in the automotive field to detect gas components in the exhaust gas of an internal combustion machine having at least one engine. Examples of such sensor elements are described in Robert Bosch GmbH: Sensoren im Kraftfahrzeug [Sensors in Motor Vehicles], 2007 edition, pages 154-159. The sensor elements described therein may also in principle be operated as per a method according to the present invention and/or used within the scope of a sensor device according to the present invention.
A lambda sensor is generally based on the use of at least one galvanic oxygen concentration cell having at least one solid electrolyte. Alternatively or additionally, so-called pump cells may also be used. Lambda sensors may have a single cell or also a multi-cell construction, reference also being able to be made to the cited related art as an example. Such sensor elements generally have at least one heating device. The solid electrolyte thus generally becomes conductive to oxygen ions at an activation temperature of approximately 350° C. The nominal temperature of normal lambda sensors is generally significantly higher, for example, at 650° C.-850° C. In order to achieve the nominal temperature independently of the ambient conditions, for example, the temperature of the exhaust gas, the sensor element is generally actively electrically heated. For this reason, most sensor elements of the mentioned type have at least one electrical heating element, which is also referred to hereafter in general as a heating device and which is generally activated by at least one control unit. For example, known lambda sensors based on zirconium dioxide have an integrated platinum heater, which is generally designed in such a way that it has a greater heating power reserve under normal operating conditions. This means that the heater voltage or heating power required for the operation of the sensor element is generally significantly lower than the available supply voltage or supply power. For example, in typical sensor elements of the above-mentioned type, operating temperatures of 780° C. are already reached at heater voltages of less than 8 V. In many cases, the sensor element is not operated using a DC voltage, but rather using a clocked effective voltage, which is generated by pulse width modulation of a higher DC voltage (battery voltage). Accordingly, the concept of a heater voltage may be understood hereafter as both the actual voltage, which is applied to the heating device, and also alternatively an effective voltage.
The output signal of a sensor element of the above-mentioned type is generally functionally heavily dependent on the temperature of the sensor element. To improve the signal accuracy, decoupling the temperature of the sensor element from changes in the exhaust gas temperature and keeping it as constant as possible are therefore to be sought. For example, a temperature control of the heater voltage of a discrete-level sensor via a characteristics map as a function of an operating point using the input variables of an exhaust gas temperature and an exhaust gas mass flow rate is routine. Increased temperature accuracy results through a temperature regulation of the sensor element. For example, an internal resistance Ri of the sensor element may be used as the controlled variable, for example, of at least one cell of the sensor element, since there is generally an unambiguous relationship between the internal resistance and the temperature of the sensor element. For example, in commercially available discrete-level sensors, an internal resistance of 220Ω corresponds to a sensor element temperature of 780° C. A corresponding temperature regulation is also used in broadband sensors.
In spite of the improvement of the signal accuracies through existing regulations, demand and improvement potential still exist for more exact temperature settings, to further improve the signal accuracy of the sensor element. In particular, the operating temperature of the sensor element is to be settable independently of the exhaust gas temperature, to further increase the signal accuracy and therefore in turn allow lower emissions and more robust diagnoses. At the same time, an activation having a high heating power reserve is to be made possible, without the risk existing of destroying the sensor element by overheating upon heating in the fundamental additional vehicle electrical system voltage range. Furthermore, the sensor element of an exhaust gas sensor is to be operable at the most constant possible temperature and is to be protected from overheating at the same time.